In Chrome, you can right-click on a search field and add it as a search engine.
You can then go into your settings and edit the search engine. For example, I might add the terms below so that if I enter the name of a country, I am sent to that country’s entry in the World Factbook.
site:https://www.cia.gov/the-world-factbook/countries/ !ducky
This process lets me add search engines and customize them in any Chromium-based browser.
With regard to customization, Focus is similar to Chrome because I can add terms to my search and reuse those searches later. Right now, I can add the equivalent of site: to Focus engines. But, hopefully, there will be more customization available in the future.
In my case, I use Firefox and Vivaldi on the desktop. And I use Safari on an iPad.
All three of these behave differently.
Vivaldi is the most customizable. I can create custom search engines like my example above.
Firefox is less flexible because a site has to publish an OpenSearch plugin before I can add it as a search engine. And those engines can’t be customized.
Mobile is the least flexible because I have a fixed set of search engines to choose from. And I can’t add to those. For example, if I want to search using Mojeek, I have to visit mojeek.com.
Ultimately, Focus does not let me do anything I could not do before. With enough typing and bookmarking, I can do all of my customized searches everywhere.
What I thought was potentially useful about Focus was that I had the same interface across browsers and platforms. And Focus makes it easy to customize searches for Mojeek. For example, this is the first time that I’ve had customized searches in Firefox.
The potential benefits of Focus make sense to me because I’m using three different browsers in three different ways. I can imagine that if you just use Google and don’t do any advanced searching then you won’t understand or care about this view.
So the answer to your question is that Chromium-based browsers allow their searches to be customized similar to what Focus can do now. And, if you view Focus as a unified interface, then Focus adds customized searches to Firefox and mobile. That is how I arrived at my comment.
Let me know if this makes sense.